The present invention relates to a phosphor screen for detecting radiation, particularly X-rays, utilizing phosphors disposed in microchannels disposed in a planar substrate.
Fine detail visualization, high-resolution high-contrast images are required for many X-ray medical imaging systems and particularly in mammography. The resolution of X-ray film/screen and digital mammography systems is currently limited to 20 line pairs/mm and 10 line pairs/mm, respectively. In particular, light scattering by the phosphor particles and their grain boundaries results in loss of spatial resolution and contrast in the image. In order to increase the resolution and contrast, scattering of the visible light must be decreased. The present invention is directed to a novel microchannel composite screen design, which provides high resolution, high contrast, and efficient X-ray to visible light conversion screens for X-ray imaging. The microchannel phosphor screen can be used in both electronic (digital) and film (analog) X-ray imaging.
A conventional X-ray screen, as shown in FIG. 1 herein has a thickness of about 30-300 microns (xcexcm) and consists of phosphor particles with a mean size between a few to 10 microns. The light generated in the screen by the incident X-ray diffuses towards the film emulsion over the finite thickness of the screen material. As the light diffuses, it spreads out which results in a loss of spatial resolution and contrast in the image. To improve resolution and contrast, thinner screens could be employed. However, use of the standard larger-particle phosphors in thin screens results in grainy images, poor resolution and low X-ray absorption. It is therefore necessary to significantly reduce the phosphor particle size and/or reduce light scattering to improve image resolution.
There are two well known scintillation screens for medical X-ray applications, both produced by Eastman Kodak and referred to as xe2x80x9cLanex(trademark) Regularxe2x80x9d and xe2x80x9cMin R(trademark)xe2x80x9d. These screens both comprise a layer of scintillation material disposed on a polymer layer. The Lanex regular screen has relatively high light output per unit of X-rays, but a low resolution of only about 5 line pairs per millimeter (lp/mm). The Min R screen is thinner than, and has a light output of only about 50%, of the Lanex regular screen, but a higher resolution of 12-13 lp/mm, although the higher resolution images are generally of lower contrast. The present invention provides microchannel based X-ray screens that have greater than 20 lp/mm and a light output a significant percentage of the Min R screen. As currently developed, the microchannel based X-ray detectors using 10 micron microchannels have a theoretical resolution of over 40 lp/mm and a light output of about 25% of the Min R screen (12% of Lanex regular screen).
It has been determined that the highest resolution and lowest light scattering could be achieved only if the phosphors are disposed in microchannels. When microchannel substrates are filled with phosphors, a new class of high resolution microchannel phosphor screens become available for various medical imaging applications. By proper selection of the phosphors and substrate materials, the X-ray generated light propagates in a waveguide mode by means of internal reflection, thereby significantly reducing scattering. Thus, the microchannel screen of the present invention can dramatically enhance contrast and resolution and ensure more accurate detection and better diagnostic imaging capabilities.
Our previous work in the design and construction of microchannel based X-ray screens can be found in U.S. Pat. No. 5,952,665; issued Sep. 14, 1999 Entitled xe2x80x9cComposite Nanophosphor Screen for Detecting Radiationxe2x80x9d; U.S. patent application Ser. No. 09/385,995 filed Aug. 30, 1999 Entitled xe2x80x9cMicrochannel High Resolution X-ray Sensor Having an Integrated Photomultiplierxe2x80x9d, U.S. patent application Ser. No. 09/197,248 filed Nov. 20, 1998 Entitled xe2x80x9cComposite Nanophosphor Screen For Detecting Radiation Having Optically Reflective Coatingsxe2x80x9d and PCT published application No. WO 99/28764. The disclosures of these previous US patent applications and issued patent is hereby incorporated by reference as if fully set forth herein.
It has also been determined that phosphor filled microchannel plates for radiation detection should use a geometry and construction optimized for the particular type of use to which the plate is to be put. A number of prior art patents have proposed to fabricate high resolution X-ray screens, by cutting and/or ablating, standard scintillation screens and filling in the cuts with reflective or low refractive index material. Other approaches in the prior art have utilized microchannel plates filled with phosphors. However, without a systematic approach and an optimization of the microchannel""s and phosphor""s construction and dimensions and the relationship between the microchannels and the phosphors, the plates will not emit any useful amount of light. If the plate design and construction is not optimized, the visible light output will be severely compromised. Indeed it is possible that, without proper design, almost all (≈99%) of the visible light generated by the phosphors will be lost in the microchannels, thus rendering the plates useless even though the phosphors efficiently converted the absorbed X-ray energy to visible light.
In order to correctly design a microchannel plate for radiation detection a critical design feature is that the maximum amount of the light generated by the phosphors is directed to the light sensor and not in any other direction. In this regard, the microchannel plate, the phosphors and the relationship between the microchannel and phosphors dimensions must be carefully considered. Regarding the microchannels, their walls must sufficiently smooth so that they reflect the light down the microchannels towards the light sensor and not in random directions and such that they will be capable of being coated with a highly reflective coating. A less than highly reflective coating will also cause considerable loss of the light impinging on it.
Loss of light generated by the phosphors in the microchannels can also be attributed to the scattering of the generated light by xe2x80x9cscattering surfacesxe2x80x9d associated with the phosphor particles. The phosphors provide numerous scattering surfaces if their dimensions, their size relative to the size of the microchannels, and their size distribution are not carefully controlled, so that the phosphors located further down the channel will not scatter the light generated by the phosphors above them. The size and dimensions of the microchannels and the phosphors themselves must also be correlated with the X-ray energy falling upon the plate so that the optimum light output is delivered to the light detector. Light scattering by the phosphor particles can also be minimized by the use of index matching materials disposed in the microchannels with the phosphors. Each microchannel can be analogized to an optical fiber in that each parameter of the optical fiber, and the microchannels and phosphors used herein, should be arranged so that the light is guided towards the light sensor with minimal loss.
A goal of the present invention is to provide a high resolution high contrast X-ray screen which can be used both for analog (film) and digital systems. The concept of a microchannel based phosphor plate provides that X-ray radiation can be measured in both digital and analog mode with similar resolution. Several limitations in the existing systems such as loading factors, sensitivity, contrast and resolution are optimized and improved significantly in the proposed system. A portable X-ray imaging system capable of digital, large area imaging for teleradiology can be built. Such an X-ray imaging system would, for example, include an X-ray generator, a microchannel based phosphor screen, a built-in detector or a CCD or CMOS camera, processing electronics and a high resolution display.
In addition to integration with light collection devices such as CCD/CMOS detectors, the microchannel phosphor screen of the present invention can also be integrated with a photomultiplier array to greatly increase the light output of the microchannel phosphor screen. The photomultiplier offers the following advantages over the CCD/CMOS detector: 1) the ability to detect 100 to 1000 fold lower light output level from the phosphor screen; 2) an Inherently higher radiation hardened device structure and 3) higher detector speed. The use of a photomultiplier arrays enables the microchannel screen/sensor to be used in conjunction with at least an order of magnitude lower X-ray dosage; which enables realtime X-ray cine-imaging with a higher frame-rate; and the use of phosphors that have a lower light output level under X-ray irradiation.
X-ray microchannel phosphor screens constructed in accordance with the present invention provide a number of key advantages over the prior art:
1. The present X-ray system based on phosphors disposed in microchannels greatly reduces photon scattering, which results in higher resolution and contrast.
2. X-ray systems based on the present invention are compact, light weight and rugged, thus being readily portable.
3. The present X-ray systems are optimized for digital imaging, enhancement, storage and transmission.
4. The present X-ray systems provide high luminescence at low X-ray dosage levels.
5. The present X-ray systems are suitable for non medical imaging applications.
6. The present X-ray systems are suitable for CAT and 3-D tomography applications.
7. The present X-ray systems optimize the light output from the phosphors and transmission to the light detectors.